The present invention relates to a method and apparatus for detecting accurately the edge of a fine pattern on a specimen (e.g. a fine circuit pattern formed on a semiconductor device) by means of a scanning electron microscope.
The edges of fine circuit patterns formed on semiconductor devices are detected using a scanning electron microscope (SEM). The SEM projects an electron beam on to an edge portion of the pattern to detect a reflected version thereof. By processing the detected signal, the presence of the edge is detected and its feature is evaluated. An example of detecting the edge position and measuring the width of the edge portion is disclosed in Japanese Patent Application No. 56-61604. In this method, a circuit pattern is magnified on the CRT screen of an SEM, and the edge position and width are detected visually by placing a cursor on the image.
The method will be described in further detail in the following. A pair of cursors are composed electrically on the magnified image of a circuit pattern and the position of the cursors is adjusted by turning the knobs of potentiometers or the like. An operator places one cursor over the left edge of the pattern and another cursor over the right edge of the pattern while turning the knobs of the cursors. Then, the operator reads the difference of readings on the potentiometer dials as the distance of two cursors, and evaluates the actual pattern width by the calibration based on the magnification factor. Although this method is simple, the accuracy of measurement largely depends on the operator's skill, and the positioning of the cursors over the pattern edge easily may cause an error when the circuit pattern does not have a sharp edge.
Recognition of edges of a circuit pattern on the two-dimensional image on the CRT screen by human visual observation and alignment of the cursors with the edges depend largely on the contrast in the SEM image signal as shown in FIG. 1. The figure shows the waveform of a signal produced when the electron beam spot makes a scanning pass across the edge of a circuit pattern (i.e., the signal corresponds to a scanning line in the two-dimensional raster image). The circuit pattern is assumed to have an edge portion having in its cross section 1 a concave corner line 2 and a convex corner line 3. Scanning of this portion by the electron beam produces a signal shown as waveform S on a plane with its abscissa representing the scanning position x and its ordinate representing the magnitude d of the signal. The signal has a minimum peak S.sub.1 at the concave corner line 2 and a maximum peak S.sub.2 at the convex corner line 3, thereby resulting in the creation of a dark line at the concave corner line on the left of the slant of the edge and a bright line at the convex corner line on the right of the slant in the two-dimensional image. In measuring the pattern width, an operator places the cursors over the bright and dark lines (for measuring the bottom width of a pattern, two cursors are placed over dark lines on both side of the pattern, or for measuring the pattern width at its top flat portion, the cursors are placed over the bright lines).
The signal waveform S of FIG. 1 suggests that the positions of the edge corner lines can be accurately detected by detecting positions providing the maximum and minimum peaks of the signal. However, noises are included in the signal from the SEM. On this account, because the simple peak value detection can easily invite a detection error, and also because coincidence of the position of the peaks with the corner lines is not guaranteed although they qualitatively have the relationship as shown in FIG. 1, the detection error is unavoidable for a possible automatic detection system. The detection error is large relative to the resolution of the SEM, posing a difficult problem in the attempt to detect the edge position accurately using an SEM.